This application builds on a foundation of recent results from our laboratory detailing a general strategy for the design of beta-peptides that are highly 14-helical in water and bind with high affinity to protein surfaces, such as the p53AD interaction domain of hDM2. In Aim 1 we explore the structural features that contribute to 14-helix stability in water and the ability of the 14-helical beta-peptide beta53-1 to recognize protein surfaces. We will complete an extensive "host-guest" analysis that will classify all proteinogenic and selected non-proteinogenic side chains as 14-helix stabilizing, destabilizing, or neutral, generating a database of position-dependent, 14-helix propensities in water; determine the NMR solution structure of beta53-1 to support the host-guest data and guide future design efforts; and explore whether the stability or affinity of beta53-1 can be improved by introduction of cyclic ACHC residues. In Aim 2 we build on the results of Aim 1 to design beta-peptide ligands for the envelope glycoproteins of 3 viruses that threaten human health, national security, or both: HIV, human respiratory syncytial virus (HRSV), and the coronavirus that causes severe acute respiratory syndrome (SARS-CoV). The experiments in Aim 2 will validate our betapeptide design strategy in a system that is highly relevant and tractable, and will likely provide leads for future drug development. In Aim 3 we develop methods to synthesize, analyze, and screen beta-peptide combinatorial libraries, and use them to optimize the affinities (and minimize the size) of beta-peptides identified in Aims 1 and 2. This aim also includes an experiment to identify cell-permeable beta53-1 library members, information that will guide design of beta-peptide ligands for additional validated targets. Taken together, the experiments in this application will provide fundamental information on ligand design and help achieve 1 of the most central and critical (yet unmet) goals of chemical biology research, the rapid identification of high affinity ligands for the vast array of potential non-enzymatic protein targets.